Antennae for transmission of radio signals are currently designed to match the impedance of the transmitter as well as possible for efficient use of transmitter power.
Antennae for radio reception also benefit from good impedance match to the receiver input, but efficiency is less of a concern here as receiver gain is relatively inexpensive to obtain. Nonetheless, in high frequency reception electric dipoles resonating at the frequency desired or the center of a band of frequencies to be received are widely used. For broadcast band reception and direction finding loop antennae are often used, tuned to the frequency of the desired signal. Loop antennae are rarely used with transmitters.
Presently, efficient antennae require that the ratio of radiator size to half wavelength not be much smaller than one. In the main, the size-to-wavelength ratio determines the phase difference between the antenna driving voltage and current. Resonant radiation occurs when the two are in phase; phase difference approaches 90.degree. in existing antennae as the size-to-wavelength ratio decreases.
In existing antennae, the radiation pattern and input impedance are, respectively, weak and strong functions of the size-to-wavelength ratio. The spatial and time variation of currents on linear antennae are described by mathematical sums over an infinite number of linear electric multipoles. Relative multipolar moment magnitudes are strongly dependent upon size-to-wavelength ratio. Relative magnitudes, in turn, determine the input impedance and pattern.
Although several current antenna designs operate over a large frequency range, their fixed dimensions impose bandwidth restrictions. An example is an equiangular spiral structure where the conductors are shaped in a spiral pattern, describable as: EQU r=Ce.sup.a(.phi.-p) ( 1)
where r and .phi. are the range and angle coordinates in a spiral system and C, a and p are constants. The spacing of the input terminals determines the high frequency limit, and the outer radius of the spiral arm determines the low frequency limit. The antenna responds to input signals at all frequencies between the two extremes.